1. Field of the Invention
This invention relates to a semiconductor laser device achieving an incoherent length for use as a light source in video discs, analog optical communication, fiber-gyros, etc., and a method for driving the same.
2. Description of the Prior Art
In general, semiconductor laser devices which function not only to record signals on optical discs but also to read them are of a DRAW (direct read after write) type. These semiconductor laser devices are required to achieve as high an output power operation as possible so as to make recording of signals easy. For this purpose, the front facet for emitting light therefrom is coated with a low-reflection index film and the rear facet is coated with a high-reflection index film, so that a large amount of light can be emitted from the front facet, thereby achieving high output power operation. This approach is advantageous in that the differential efficiency from the front facet is improved and high output power can be produced by a small amount of driving current. However, the reflection index of the light-emitting facet of such a laser device is so low that light reflected by an optical disc returns to the laser device and attains an optical coupling with light produced within the laser device, resulting in an external resonator positioned between the front facet of the laser device and the optical disc face. The length of the external cavity varies to a great extent with movement of the optical disc face, which causes reflected light-induced noise, resulting in errors in reading the signals, which makes serious problems in practical use. In order to prevent the reflected light-induced noise, a method by which the coherent length of a semiconductor laser device is shortened enough so that the formation of an external resonator between the optical disc face and the laser device does not occur is known. According to this method, gain guided laser devices or index guided laser devices with a lowered refractive index are used so that laser oscillation in a multi-longitudinal mode can be obtained or the width of each element of the multi-longitudinal mode can be enlarged by self-pulsation to thereby shorten the coherent length, causing the instability of the transverse mode, which makes high output power operation difficult.